The present invention relates to the treatment and disposal of bauxite residue or so-called "red mud" which is generated in the Bayer Process in the production of alumina (Al.sub.2 O.sub.3). The alumina so produced may be further processed to yield aluminum metal or it may find use directly in numerous non-metallurgical applications.
In the Bayer process, alumina is extracted from naturally occurring bauxite ore. The bauxite is digested in a digester vessel at elevated temperatures and pressure with a strong solution of caustic soda (NaOH). In the presence of a strong base, the alumina acts as an acid to form sodium aluminate in solution. The impurities remain insoluble and form the red mud. The red mud is then separated from the green liquor solution by decantation and filtration. The green liquor formed at the high digestion temperature is cooled and becomes supersaturated at lower temperatures. The cooled liquor is then inoculated in a precipitator vessel with aluminum hydroxide crystals, Al(OH).sub.3, and agitated. A portion of the alumina gradually crystallizes and precipitates out of solution in hydrated form and is separated from the spent caustic liquor by filtration. After washing, the hydrate is calcined to the oxide form to provide a very pure alumina. It may also be collected and sold in the hydrate form. The spent caustic liquor and wash water, after concentration, are recycled and used for another digestion treatment of bauxite ore.
Conventionally, the red mud residue is removed from the digested slurry of bauxite and caustic soda/sodium aluminate liquor using a plurality of hydraulic thickeners/washers or pressure filters. Hydraulic thickeners are large vessels on the order of 100 feet in diameter, which require large volumes of water to achieve proper treatment and thickening of the red mud residue. These thickeners rely heavily upon the settling characteristics of the residue and are known to suffer major upsets when this characteristic changes. The large volumes of water associated with the thickeners must also be diminished by steam heating/evaporation which necessarily increases energy consumption and capital costs associated with the equipment and energy usage. These large vessels also require periodic cleaning of the dried red mud residue which, in itself, is a difficult and time-consuming task requiring shutdown of the thickener to remove the hardened residue over these very large surface areas.
Conventional pressure filters, on the other hand, rely on the filtration characteristics of the red mud. Such filtration characteristics may often change abruptly and without advance warning. Further, few bauxite ores produce a red mud residue possessing consistently good filtration characteristics. Such bauxite ores naturally occur in only specific bauxite deposits throughout the world and long term supply of such ores is oftentimes problematic.
Heretofore, it has been conventional practice to dispose of treated, i.e., washed and thickened red mud by impoundment in waste ponds or lakes. In such impoundment lakes, the red mud settles out by gravity and the supernatant liquor is returned to the process for reuse, such as hose water, wash water and the like, or it may be disposed of after proper treatment. As the red mud settles, it entrains about 45% by weight lake water which may contain, for example, up to about 35 grams per liter of caustic soda and 8 grams per liter of alumina which was present in the liquid portion of the red mud slurry when it was pumped to the impoundment lake. Thus, considerable quantities of caustic soda and alumina are lost to occlusion in the red mud impoundment lakes which represents a significant monetary loss. Aside from these chemical losses, the lake water itself poses a disposal problem from an environmental standpoint due to a high alkalinity. Groundwater runoff is not permissible unless the caustic lake water is neutralized.
In addition to the caustic and alumina losses and the negative environmental impact, conventional red mud impoundment lakes demand substantial land usage on a continuing basis. For example, a typical impoundment lake may occupy about 50-70 acres or more which is non-recoverable land and is useful for only about five years. In addition to the land costs, significant costs are realized in constructing and maintaining the dikes which retain the waste lake water and red mud.
The present invention solves the problems heretofore encountered in the treatment and disposal of red mud bauxite residue. The instant invention provides methods for treating red mud which, in one aspect, replaces the pressure filters of the prior art. Thus, the present invention eliminates the dependence upon the use of only a limited number of bauxite ores which heretofore had to be used in conjunction with pressure filters due to their consistent filtering characteristics. The present invention eliminates the need for pressure filters and their dependence upon such "good filtering" bauxite, which not only increases productivity but also permits the use of many other sources of bauxite ores which may be poorer in filtering characteristics.
The present invention also eliminates the need for numerous washing steps and large-sized thickener vessels presently used in the art. The invention thus provides a significant reduction in physical plant space required to house the apparatus for treating red mud, while also reducing the need for large volumes of wash water previously required in such existing process systems. Because the present invention minimizes the use of wash water, there is no need to construct extra evaporative heating capacity as is necessary in these conventional treatment systems.
Still further, the process of the present invention provides a treated red mud which contains minimal amounts of entrapped caustic and alumina. Concurrently, the wash water liquor recovered in the present invention contains significantly greater amounts of caustic and alumina values than previously recovered, and in an economical manner. The additional caustic value recovered from the red mud is then reused in the digestion process saving the expense of extra caustic additions heretofore required. Further, the higher levels of alumina values recovered from the red mud is recovered to provide a further economic advantage over conventional processes.
The present invention further includes a method for disposing of the treated red mud in a so-called "dry stacking" mode which eliminates the need and attendant expense of constructing and maintaining waste lakes. Capital costs for the apparatus employed in the dry stacking aspect of the invention are also lower than conventional methods.